Green Alternatives to Zinc Dialkyldithiophosphates: Vanadium Oxide-Based Additives

A functionalized vanadyl(IV) acetylacetonate (acac) complex has been found to be a superior and highly effective antiwear agent, affording remarkable wear protection, compared to the current industry standard, zinc dialkyldithiophosphates (ZDDPs). Analysis of vanadium speciation and the depth profile of the active tribofilms by a combination of X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) analyses indicated a mixed-valence oxide composite, comprising V(III), V(IV), and V(V) species. A marked difference in composition between the bulk and the surfaces of the tribofilms was found. The vanadyl(VI) acac precursor has the potential to reduce or even replace ZDDP, which would represent a paradigm shift in the antiwear agent design. A major benefit relative to ZDDPs is the absence of S and P moieties, eliminating the potential for forming noxious and environmentally harmful byproducts of these elements.


VO(C8-acac
Single crystal X-ray diffraction data was collected at 150 K using either an Agilent Xcalibur or Agilent SuperNova Dual diffractometer with either Mo-Κα (λ = 0.71073 Å) or Cu-Κα (λ = 1.5418Å) radiation.Structures were solved by fill-matrix least squares refinement using either the WinGX-170 suite of programs or the X-SEED programme suite.

Preparation of Oils
A 4 cSt group III mineral oil and a model ZDDP were provided by Infineum UK Ltd.Fully formulated oils used in this study are commercially representative model systems, with analytics as presented in table S3.Lubricant samples for testing were prepared by blending 10 mmol of additive into either base oil, FFO1 or FFO3 at 60 °C with stirring.ZDDP and MoDTC in the model samples were at commercially representative concentrations.

Tribological Testing
The lambda value (λ) was determined using equation S1.1. Where: And the minimum film thickness can be given by: Where R x is the radius of curvature in the x direction, U is the entrainment speed, E* is the reduced Young's Modulus, η 0 is the dynamic viscosity of the lubricant, α is the pressure-viscosity coefficient, W is the applied load and k = 1.The entrainment speed is given by the following: The reduced Young's Modulus is given by: Where υ 1 and υ 2 are the Poisson's Ratio of each material and E 1 and E 2 are the Young's Modulus of each material.

Wear Measurements
The worn volume of the pins has been measured via 3D white light interferometry profilometry, using a non-contact NPFLEX.
The worn volumes were converted into dimensionless wear coefficients, K, via Archard's equation: Where V is the worn volume (mm 3 ), H is the Brinell Hardness of the sample (7450 Nmm -2 ), P is the normal load ( 50 S1.S3, are provided as references.S3, are provided as references.

Figure S3 .
Figure S3.Wear coefficient as a function of additive concentration for compounds 1-3 in base oil.Data collected using TE 77 reciprocating pin-on-plate tests of each additive at a contact pressure of 250 MPa.Corresponding coefficient of friction measurements displayed in Figure S6.Data for base mineral oil is provided as reference.

Figure S4 .
Figure S4.Wear coefficient as a function of additive concentration for compounds 1-3 in base oil.Data collected using TE 77 reciprocating pin-on-plate tests of each additive at a contact pressure of 1 GPa.Corresponding coefficient of friction measurements displayed in Figure S7.Data for base mineral oil is provided as reference.

Figure S5 .
Figure S5.Coefficient of friction as a function of additive concentration for compounds 1-3 in base oil.Data collected using TE 77 reciprocating pin-on-plate tests of each additive at a contact pressure of 250 MPa.Corresponding wear coefficient measurements displayed in Figure S4.Data for base oil is provided as reference.

Figure S6 .
Figure S6.Coefficient of friction as a function of additive concentration for compounds 1-3 in base oil.Data collected using TE 77 reciprocating pin-on-plate tests of each additive at a contact pressure of 1 GPa.Corresponding wear coefficient measurements displayed in Figure S5.Data for base oil is provided as reference.

Figure S7 .
Figure S7.Wear coefficient as a function of additive concentration for compounds 1-3 in a commercially representative oil without ZDDP and MoDTC loading (FFO-1).Data collected using TE 77 reciprocating pin-on-plate tests of each additive at a contact pressure of 250 MPa.Corresponding coefficient of friction measurements displayed in Figure S10.Data for commercially representative oils FFO-1 and FFO-3, defined in TableS3, are provided as references.

Figure S8 .
Figure S8.Wear coefficient as a function of additive concentration for compounds 1-3 in a commercially representative oil without ZDDP and MoDTC loading (FFO-1).Data collected using TE 77 reciprocating pin-on-plate tests of each additive at a contact pressure of 1 GPa.Corresponding coefficient of friction measurements displayed in Figure S11.Data for commercially representative oils FFO-1 and FFO-3, defined in TableS3, are provided as references.

Fig S11 .
Fig S11.EDX images of the cross-section of a tribofilm formed from 3. A TEM image of the corresponding tribofilm is in figure 4.

Figure S12 .
Figure S12.Comparison of the relative oxidation state contributions inside and outside the wear scar of a tribofilm formed from 3, as determined by XPS.

Figure S13 .
Figure S13.Scanning X-ray fluorescence (XRF) map of the wear scar used to identify the wear scar location by virtue of the high localised concentration of vanadium.Subsequent measurements "inside" the wear scar are obtained within the red region, and "outside" in the blue region.

Figure S14 .
Figure S14.XANES spectra from A) inside and B) outside the wear scar, overlayed with relevant vanadium oxide reference data.Inset: Derivatives of the XANES spectra.

Figure S15 .
Figure S15.Comparison of the vanadium oxide peak positions inside and outside the wear scar with relevant vanadium oxide references.See Figure S13 for the X-ray fluorescence map used to identify areas inside and outside the wear scar.